AWS DMS issue with source endpoint in EC2-classic

I had the same issue with the same error message and it was a network issue. My replication instance didn't have access to the database.

My database is in a VPC with a subnet x and my replication instance is in the same VPC with the same subnet x. I opened the 3306 port in my Network ACL and in the security group of the database to the Internet (0.0.0.0/0) just to test if it was a network problem. The connection test worked with those settings. After the test, I removed the last setting for security.

The solution to my problem was to open the 3306 port in the security group and in the network ACL of the database to all the vpc connection. e.g. 172.0.0.0/16.

Hope it helps

You must add the security group from your DMS replication instance to your RDS database associated security group as an authorised inbound traffic.

• Go to Database Migration Service dashboard and them to "Replication Instances"
• Select your replication instance to get the "VPC Security Group"